1. Field of the Invention
The present invention relates to an apparatus and method for random access in a wireless communication system. More particularly, the present invention relates to an apparatus and method for ranging in a broadband wireless communication system.
2. Description of the Related Art
As known in the art, communication systems were originally developed to provide voice communication services. As technology has evolved, communication systems now provide data services and various multimedia services in addition to the voice communication services. However, conventional communication systems, which were developed to provide only voice communication services, have a narrow bandwidth for data transmission and require a high subscription fee, thus failing to satisfy diversified user demands. Furthermore, in conjunction with the rapid development in the communication industry and ever-increasing demands for Internet services, it is important to provide communication systems capable of providing Internet services efficiently. As a result of these trends, broadband wireless communication systems for providing efficient Internet services have been proposed.
The proposed broadband wireless communication systems use an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme. Thus, the broadband wireless communication systems can provide high-rate data transmission by transmitting physical channel signals using a number of subcarriers. The standardization of wireless access schemes for broadband wireless communication systems is being conducted by the Institute of Electrical and Electronics Engineers (IEEE), which is an international standardization organization, particularly by the IEEE 802.16 standardization group.
Ranging is a kind of random access process in a broadband wireless communication system. The ranging process is used to correct time/frequency offsets between a Base Station (BS) and a Mobile Station (MS) and to correct the transmission (TX) power of a mobile station. The ranging process can be divided into initial ranging, periodic ranging, bandwidth request ranging, and handover ranging, depending on the purpose.
A ranging code (or code set) used in each ranging process is broadcast using an Uplink Channel Descriptor (UCD) message. A mobile station attempts ranging by transmitting an intended ranging code through a ranging region that is allocated using an UpLink (UL)-MAP message. A base station uses the received ranging code to calculate time/frequency offsets, and transmits a RaNGing ReSPonse (RNG-RSP) message containing an offset correction value to the mobile station. That is, through a code ranging process, the mobile station corrects physical time/frequency offsets with the base station. The ranging code transmitted by the mobile station in the ranging process is called a Code Division Multiple Access (CDMA) code. The CDMA code is generated by a Pseudo Random Binary Sequence (PRBS) generator illustrated in FIG. 1.
FIG. 1 is a diagram illustrating a conventional ranging code generator, i.e., a PRBS generator.
Referring to FIG. 1, a polynomial of a PRBS generator is (1+X1+X4+X7+X15), and a CDMA code represents a 144-bit binary sequence generated by the PRBS generator. An initial value (i.e., a seed) of the PRBS generator, i.e., B14˜b0 is (0.0.1.0.1.0.1.1.s0,s1.s2.s3.s4,s5,s6), and a specific parameter (i.e., UL_permBase) of a corresponding base station is set for s6:s0. The initial 144-bit sequence output from the PRBS generator is defined as the 0th CDMA code, and the next 144-bit sequence constituted by the subsequent 145th to 288th bits is defined as the first CDMA code. In general, each base station manages 0˜25 codes, classifies the 256 codes into several subgroups depending on uses, and broadcasts the use of each subgroup through a UCD message.
The uplink synchronization of a mobile station may deviate when its distance from a base station changes. Also, the uplink synchronization may deviate due to a handover situation or due to the inaccuracy of an oscillator used by the mobile station. Periodic ranging is a process for correcting such a synchronization deviation. In periodic ranging, the mobile station transmits a CDMA code to the base station at preset periods, and corrects a synchronization deviation using a ranging response (RNG-RSP) message received from the base station. For example, the ranging status set in the ranging response message may be SUCCESS, CONTINUE, or ABORT, depending on the degree of an uplink synchronization deviation. If the ranging status is SUCCESS or CONTINUE, the ranging response message may contain correction values of timing, frequency and TX power. Also, because it is difficult to know the position of the mobile station that has transmitted the CDMA code, the ranging response message is encoded using a robust coding scheme so that the mobile station at a cell edge also can receive the ranging response message.
The existing system uses a fixed ranging period without considering the movement of a mobile station or the degree of any synchronization deviation. Periodic ranging aims at achieving uplink synchronization. Therefore, an increase in the frequency of the periodic ranging increases the accuracy of synchronization with a base station and stabilizes the throughput of transmission of user traffic, thus making it possible to expect a satisfactory Quality of Service (QoS). However, if the periodic ranging is performed too frequently, the probability of contention in a ranging region increases, thus interrupting the operations of other mobile stations and wasting radio resources due to the occupation of resources by a ranging response message. On the contrary, if the period of the periodic ranging increases, the probability of a deviation in the uplink synchronization of the mobile station increases, thus causing a delay in the mobile station's ability to transmit traffic until synchronization is acquired.